How hot is too hot for humans?

Record-setting heat waves have gripped the U.S. only weeks into summer, and at least 38 people are suspected to have died from heat-related issues so far this summer. 

Climate change is fueling new heat extremes, and researchers who are racing to better understand the limits of human survival are increasingly focused on determining at what temperature thresholds people can survive without air conditioning and how soon some parts of the world could reach deadly levels. 

Some scientists worry that a widespread heat wave could strike a region without reliable air conditioning and cause a mass death event. And recent research suggests that the threshold for surviving heat is far lower than what some scientists once thought, particularly for older people and in dry climates.

“We can only sweat so much, and older people even less,” said Jennifer Vanos, an associate professor in the School of Sustainability at Arizona State University. 

“Where we live in the U.S., heat deaths should be 100% percent preventable. There should be air conditioning in some location. But, in some parts of the world, that doesn’t exist.” 

Here’s what to know: 

Keep the core cool

The human body can’t tolerate its temperature reaching 43 degrees C (about 109.4 degrees F).

“Anyone who reaches that core temperature — 99.9% would die,” Vanos said.

This is the upper limit of survival. Heat often kills in more subtle ways — by worsening pre-existing issues, like cardiovascular or renal disease. 

Humidity can be key

Humans keep their core cool by sweating and shedding heat produced by their bodies.

“If it’s too humid, your sweat doesn’t evaporate, and then the outside of your skin can’t get cooler than the inside of your skin,” said Scott Denning, a climate scientist and professor of atmospheric science at Colorado State University. “Metabolic heat builds up inside your body and leads to a very high fever and heat stroke and death.”